A cathode ray tube (CRT), in general, includes a glass tube having a relatively narrow neck at one end and a funnel section ending in an enlarge face plate at the other end. A screen is on the inner surface of the face plate, and an electron gun is in the neck. For a color CRT, the electron gun generates three separate electron beams which are aligned along a plane with one beam being between the other two beams. The beams are directed against the screen on the face plate to generate a picture. Between the gun and the face plate and surrounding the tube is a yoke of a magnetic winding which control the movement of the beam over the face plate.
In the manufacture of a CRT, it is necessary to adjust the position of the yoke along the tube so that the electron beams are properly directed to their respective pixels on the screen. This adjustment is carried out by a yoke adjust moving (YAM) step. In the traditional way of carrying out the YAM, if the CRT's longitudinally axis is along the North-South direction with the North-South earth magnetic field coming from the rear end of the CRT and out the front face, a transversal moving component of each electron side beam (the blue or red beam), will experience the earth's magnetic field. As shown in FIG. 1, this results in the beams having an opposite up-down departure when they hit the screen according to the Lorentz Law. As shown in FIG. 1, one of the beams, such as the blue beam, is moved upwardly with respect to its normal path, whereas the other beam, such as the red beam, is moved downwardly. However, if the earth's magnetic field is coming from the front face to the rear end of the CRT, the misconvergence is the converse about the beam positions. As shown in FIG. 2, under this condition, the one beam, such as the blue beam, is moved downwardly from its normal path whereas the other beam, such as the red beam, us moved upwardly. Therefore, it would be desirable to provide the tube with means for overcoming this misconvergence so that the beams will be directed properly on their pixels of the screen.